test: add test-transaction-rollback-on-sigpipe.t demonstrating py3 regression

When an hg push is interrupted with C-c, the remote [hg serve] command
receives SIGPIPE.

If a pretxnchangegroup hook fails, the remote hg then tries to
rollback the transaction. It begins by printing "transaction
abort!\n". This returns EPIPE, but ui.py ignores that error.

In python3 (but not python2), this "transaction abort!\n" message
stays in a buffer, so future flushes of stderr will try to print the
message again, and so those flushes will also hit EPIPE.

This test demonstrates such a case where this EPIPE causes the
transaction rollback to fail, leaving behind an abandoned transaction.

Differential Revision: https://phab.mercurial-scm.org/D9151

tests/test-transaction-rollback-on-sigpipe.t

@@ -0,0 +1,67 b''
+Test that, when an hg push is interrupted and the remote side recieves SIGPIPE,
+the remote hg is able to successfully roll back the transaction.
+
+  $ hg init -q remote
+  $ hg clone -e "\"$PYTHON\" \"$TESTDIR/dummyssh\"" -q ssh://user@dummy/`pwd`/remote local
+
+  $ check_for_abandoned_transaction() {
+  >     [[ -f $TESTTMP/remote/.hg/store/journal ]] && echo "Abandoned transaction!"
+  > }
+
+  $ pidfile=`pwd`/pidfile
+  $ >$pidfile
+
+  $ script() {
+  >     cat >"$1"
+  >     chmod +x "$1"
+  > }
+
+On the remote end, run hg, piping stdout and stderr through processes that we
+know the PIDs of. We will later kill these to simulate an ssh client
+disconnecting.
+
+  $ killable_pipe=`pwd`/killable_pipe.sh
+  $ script $killable_pipe <<EOF
+  > #!/bin/bash
+  > echo \$\$ >> $pidfile
+  > exec cat
+  > EOF
+
+  $ remotecmd=`pwd`/remotecmd.sh
+  $ script $remotecmd <<EOF
+  > #!/bin/bash
+  > hg "\$@" 1> >($killable_pipe) 2> >($killable_pipe >&2)
+  > EOF
+
+In the pretxnchangegroup hook, kill the PIDs recorded above to simulate ssh
+disconnecting. Then exit nonzero, to force a transaction rollback.
+
+  $ hook_script=`pwd`/pretxnchangegroup.sh
+  $ script $hook_script <<EOF
+  > #!/bin/bash
+  > for pid in \$(cat $pidfile) ; do
+  >   kill \$pid
+  >   while kill -0 \$pid 2>/dev/null ; do
+  >     sleep 0.1
+  >   done
+  > done
+  > exit 1
+  > EOF
+
+  $ cat >remote/.hg/hgrc <<EOF
+  > [hooks]
+  > pretxnchangegroup.break-things=$hook_script
+  > EOF
+
+  $ cd local
+  $ echo foo > foo ; hg commit -qAm "commit"
+  $ hg push -e "\"$PYTHON\" \"$TESTDIR/dummyssh\"" --remotecmd $remotecmd 2>&1 | grep -v $killable_pipe
+  pushing to ssh://user@dummy/$TESTTMP/remote
+  searching for changes
+  remote: adding changesets
+  remote: adding manifests
+  remote: adding file changes
+  abort: stream ended unexpectedly (got 0 bytes, expected 4)
+
+  $ check_for_abandoned_transaction
+  Abandoned transaction!